Understanding Doppler Physics in Clinical Practice
Doppler is not just color.
It is math.
And math is unforgiving.
1️⃣ The Doppler Equation (쉽게 설명 버전)
Measured velocity depends on:
- Blood velocity
- Frequency
- Cosine of the angle between beam and flow
Velocity ∝ cos(θ)
2️⃣ Why Angle Correction Matters
When the beam is parallel to blood flow (0°):
cos(0°) = 1 → Most accurate
At 60°:
cos(60°) = 0.5 → Velocity significantly reduced
Above 60°:
Error increases dramatically.
Keep Doppler angle ≤ 60° whenever possible.
Clinical Reality
🫀 Umbilical artery
If angle correction is ignored → S/D ratio unreliable
🧠 MCA PSV
Wrong angle → False suspicion of fetal anemia
🩺 Uterine artery
Inconsistent angle → Abnormal PI reading
Doppler error is not subtle.
It directly changes numbers.
3️⃣ Spectral Doppler Basics
What you see on screen:
- X-axis → Time
- Y-axis → Velocity
- Envelope → Peak flow
If angle is wrong → waveform shifts.
4️⃣ Aliasing (자주 헷갈리는 것)
Aliasing occurs when velocity exceeds Nyquist limit.
Result:
- Waveform wraps around
- Color suddenly flips
Solution:
- Increase PRF
- Lower baseline
- Use lower frequency
- Adjust scale
Aliasing is physics — not pathology.
Practical Reminders in OB Ultrasound
✔ Always align cursor with vessel direction
✔ Use angle correction properly
✔ Avoid measuring above 60°
✔ Recheck abnormal values in another plane
Numbers are only as accurate as your angle.
Quick Summary Table
| Concept | Key Point | Clinical Risk |
| Angle | Keep 60 | Velocity error |
| Cosine effect | Velocity cos | Underestimation |
| Aliasing | Exceeds Nyquist | False abnormal |
| PRF | Adjust scale | Avoid wrap |
Sonographer’s Note
Doppler looks colorful.
But behind the color is trigonometry.
When the angle is wrong,
the number is wrong.
And sometimes,
the diagnosis changes by a few degrees.
